1. Field of the Invention
The present invention relates to composite materials, used in lithium-ion batteries as anodes. In particular, the present invention is directed to composites formed with carbon materials and metal and/or metal oxide nanoparticles and anodes formed from the composites that carry high current density, has high cycle life, high reversible capacity and low irreversible capacities.
2. Description of Related Art
Lithium-ion batteries have many uses including computers, portable phones and electric cars. Today's lithium-ion batteries use carbon as an anode to store and release lithium. The carbon anodes typically have long cycle life due to the presence of a protective surface-electrolyte interface layer (SEI), which is resulted from the reactions between lithium and the electrolytes during the 1st several cycles of charge-discharge. The lithium in this reaction is obtained from reduction of some of the lithium ions originally intended for charge transfer purpose. As SEI formed, the lithium ions become part of the inert SEI layer and become irreversible, i.e, they can no longer be the active element for charge transfer. Therefore, it is desirable to use a minimum amount of lithium for the formation of an effective SEI layer.
The effectiveness of the SEI to protect the anode material depends on a number of factors. One of the important factors is the type of carbon surface. It is desirable to have a certain type of carbon surface that could use minimum amount of lithium to form SEI that is most effective in protecting the anode material.
If the bulk of the carbon material is graphite, then the reversible capacity of such anode has a theoretical of 372 mAh/g. This is based on the results of research of intercalation, which indicated that the lithium intercalated carbon has a maximum atomic ratio of lithium to carbon: 1/6.
In order to have a light battery that can be used for a long time before recharge is needed, it is desirable to increase the capacity of carbon materials to store lithium. Recent investigations indicated that some types of non-graphitized carbon anodes (such as carbon nanotube) have lithium storage capacity higher than 372 mAh/g. This may be caused by some additional types of mechanisms other than commonly known mechanism of intercalation. In most cases, these types of carbon anodes have high irreversible capacity and/or low cycle life, and are still being studied.
Another type of material that can possibly be used as an anode in lithium-ion batteries is elemental metal that alloy with lithium. Some metal oxides have also been proposed for such application. These anodes may have reversible capacity higher than the 372 mAh/g value for graphite stated above, but, again, they generally had short cycle lives. It is believed that the volume changes during lithium insertion-release and the resulting internal stress or disintegration are the reasons for their short cycle lives and low capacities. To counter such mechanical degradation, small active particles supported with less active or non-active matices have been proposed. Examples of these metal elements are Si, Sn and SnO2.
In order to have a battery for high power density applications such as electric cars, it is desirable to have an anode that can carry high current (i.e., that can quickly store and release lithium). Previous report on the current density-performance relation are few, but it is believed that current density can be higher if the resistance for the lithium ions to travel for storage/release can be reduced.
Prior art electrodes have been disclosed, but many of those disclosures have deficiencies in some ways. For example, in U.S. Pat. No. 6,007,945, by Nazri, a composite is formed by heating a mixture of carbon with metal halide in the presence of “sp elements.” However, this composite has the possibility of producing metal chloride intercalated carbon as unwanted side-products.
In U.S. Pat. No. 6,143,448, by Fauteux et al., a composite is formed by mixing carbon with a metal salt in water, followed by evaporation, heating and further treatment. The process produces a composite with high surface area which is not always preferred.
In U.S. Pat. No. 6,103,393, by Kodas et al., provides carbon-metal particles by mixing the reactant, making the mixture into an aerosol, and then heating. Every particle contains a carbon phase and a metal phase, where the carbon phase is substantially unchanged in form. In many cases, the multiphase composite does not have the desired properties.
In U.S. Pat. No. 6,007,945, by Jacobs et al., a material is formed of a solid solution of titanium oxide and tine oxide to be used as anode in a lithium-ion battery. However, the nanoparticles of the anode material reacts with the electrolyte during the charge-discharge cycles, leading to reduced long-term utility.
Even taking these examples into account, the prior art has not demonstrated a composite material that has all of the properties desired for use in an anode for lithium-ion batteries. Thus, there is a need for a new anode for lithium-ion batteries that can carry high current density, has high cycle life, high reversible capacity, and low irreversible capacities. There is also a need for a method of producing such a composite that allows for such composites to be readily produced.